Conventional state of the art propulsion systems for large civil aircraft typically include one or more gas turbine engines placed under the wings of the aircraft. However, some studies have indicated that so-called distributed propulsion, which involves having numerous smaller propulsion units preferentially arranged around an aircraft, may provide some significant benefits in terms of noise reduction and fuel efficiency when compared with the current state of the art propulsive arrangements.
One known arrangement for distributed propulsion is that used for helicopters where one or more engines is connected to two variable pitch rotors, either to two main rotors, or to a main rotor and a tail rotor, using a mechanical transmission system. Such systems typically incorporate shafts and fixed ratio gearboxes. Such systems do however include clutches to enable the engines individually to be disconnected from the rest of the system in the event that they are not producing power.
It is also known to apply mechanically synchronised systems to vertical take-off aircraft such as tilt-rotor aircraft, where the rotor speeds are mechanically synchronised. In these transmission systems the power that is absorbed by each rotor, or that is generated through gyro-rotation, is regulated by means of a collective pitch mechanism, which sets the desired pitch angle on all of the blades of the rotor. Such a mechanical transmission system might also be applied to a more conventional aircraft with distributed propulsion, where the engines are wholly or partly separated from the rotors that provide the means of propulsion.
It is also known to provide numerous electrical propulsive units which are located so as to capture and accelerate slow speed boundary layer air which has formed against the surface of the aircraft. This can lead to a significant reduction in overall fuel burn with the maximum benefit of boundary layer ingestion being achieved when the low speed flow is not mixed with the freestream flow, but is accelerated to homogeneous conditions by the propulsion system.
One option for such an arrangement is to provide one or more electrical generators powered by gas turbine engines and use the generated electricity to power the various propulsive units which are arranged around the aircraft. However, such a system is electrically complex and would require a high degree of control to keep it stable. Such control would conventionally come from power electronics. However these would be prohibitively large and heavy for the required levels of power and redundancy required for aero applications.
The present invention seeks to provide an electrical distributed propulsive system which addresses the control of an electrical distributed propulsive system.